Transport networks serve for the physical layer transport of high bitrate tributary signals. In particular, the signals transmitted over a transport network are encoded and multiplexed into a continuous bit stream structured into frames of the same length. Within this constant bitrate bit stream, the frames are repeated periodically with a frame repetition rate of typically 8 kHz and are structured according to a multiplexing hierarchy. An example of such a multiplexing hierarchy is SDH (Synchronous Digital Hierarchy, see ITU-T G.707 10/2000) where the frames are termed synchronous transport modules of size N (STM-N, where N=1, 4, 16, 64,or 256). The frames have a section overhead and contain at least one higher order multiplexing unit called virtual container VC-4. A VC-4 can either directly carry a tributary signal or a number of lower order multiplexing units like VC-12 or VC-3, which then carry tributary signals. Alternatively multiple virtual containers can be concatenated to transport a higher bitrate tributary signal.
Virtual containers are transmitted from source to sink through an SDH network and therefore represent a “logical” path through the network. The sequence of identical VCs having the some relative position in subsequent frames forms a traffic stream along that path. Each VC contains a path overhead (POH) and a payload section referred to as container a (C). The US equivalent of SDH is known as SONET (Synchronous Optical Network). Another well known transport network with similar multiplexing units is the recently defined Optical Transport Network OTN; see ITU-T G.709, 02/2001.
A very basic aspect in all types of transport networks is availability and reliability of service. In other words, a transport network must be very robust against any kind of failure and the average outage time must be very low. Hence, a transport network needs to provide the means and facilities to ensure sufficient availability. Typically, network mechanisms,which ensure this availability are distinguished in protection and restoration. The common principle of both is to redirect traffic of a failed physical link or logical path over a spare resource.
Protection is a mechanisms where an already established protection path or link is assigned to one selected high-priority path or link (known as 1+1 or 1:1 protection, depending on whether there is low priority extra traffic on the protection resource or not) or a group of n such selected paths or links (known as 1:n protection). In the case of a failure, traffic can be re-routed very fast over the previously established protection resource under the sole control of the affected network elements in typically less than 50 ms. However; this requires a protocol between the affected nodes to signal and synchronize switch-over. Protection is a high-quality service restricted to few selected premium connections, which are typically charged at a higher price. Moreover, protection requires a high amount of spare resources compared with the amount of protected resources, i.e., 100% of spare capacity in the case of 1+1 protection.
Restoration refers to a mechanism, where the network searches for restoration capacity and establishes a restoration path only after a service path failure. Rather than calculating the restoration path after failure, pre-calculated restoration routes can be used instead but the cross-connections which establish the path are performed after failure. Restoration mechanisms are more stringent in the usage of spare capacity and however, providing a masking of the failure at a lower speed, typically in the range of a few seconds as completely new paths through the network must be established.
In label switched packet networks, as opposed to transport networks, alternative label switched paths (LSPs) can already be implemented and then used in the case of a failure. The fundamental difference between transport networks and packet networks, where MPLS applies (Multi-Protocol Label Switching), is that in packet networks statistical multiplexing is used allowing over-subscription of links and that an LSP can be established without using any bandwidth. However, in transport networks, if a path is established, then by definition the full bandwidth requested by the path is consumed, independent of whether traffic is transmitted over this path or not. An LSP can be established before failure in MPLS, but not used until after failure, whereas this is not possible in transport networks.
The IETF proposal “RSVP-TE Extension for Shared-Mesh Restoration in Transport Networks” by G. Li et al, draft-li-shared-mesh-restoration-01.txt, addresses this issue and proposes an GMPLS extension where backup paths are pre-established in the management plane of the network but only activated after detection of a failure. This requires signaling messages to be exchanged in the management plane, which in a GMPLS controlled network is distributed across the entire network. This interaction of a decentralized network management is, however, a relatively slow process, which leads to restoration times in the range of at least some hundred milliseconds.
The IETF proposal “Signaling for Fast Restoration in Optical Mesh Networks” by Bala Rajagopalan et al describes an alternative restoration method for GMPLS-controlled networks that proposes a new dedicated protocol rather than an RSVP-TE extension to communicate and synchronize setup of restoration paths.
EP1294136 describes a restoration method where a closed mesh is used to protect against failures of intersecting links. In the case of a failure, affected traffic is redirected automatically along the closed mesh. Based on the received signal, an automatic reconfiguration of the I/O ports is performed in each node to match the received signal structure.
EP 1463370 A1 which is incorporated by reference herein, describes a label-based restoration method in an SDH transport network, where each path in the network is identified at least locally with an identifier called a path tag which is part of the multiplex units that represent the corresponding path and forwarding information is provided in each network element. In the case of a path failure, the source network element cross-connects the affected traffic stream to an alternate output port. When a network element receives a new traffic stream at an input port, it checks the received path tag and determines an appropriate output port based on the tag and the forwarding information and establishes an internal cross-connection between the input port and corresponding output port. The forwarding information is provided by the management plane of the transport network and represents pre-calculated restoration paths.
Another method for signaling restoration information in a mesh network is known from EP 1134922 A2, which uses pre-computed restoration paths. Each node in the network has monitoring, signaling and crossconnect functionality and databases to participate actively in real time restoration. When a node detects a path failure, it formulates a signaling message for restoring the failed path. The restoration signaling message is thereafter relayed from one node to another in the overhead or payload of signaling paths that occupy the same bandwidth that is subsequently used by the restoration path. Once a signaling message is transmitted to an adjacent node, the node that transmitted the message makes a cross-connect that replaces the signaling path with a segment of the restoration path whose set-up was requested in the transmitted signaling message.
Both of these latter restoration methods, however, do not support extra traffic on the idle restoration resources but require that a “supervisory unequipped signal” or other kind of signaling path signal is sent along idle links. This results in an inefficient usage of resources.